Intraocular lenses (IOLs) were first used as a replacement for damaged natural crystalline lenses in 1949. These early IOLs were implanted into the posterior chamber after the natural crystalline was surgically removed. The first physician to use posterior chamber IOLs as replacements for the natural crystalline lens was English RAF ophthalmologist Dr. Howard Ridley. Dr Ridley first observed acrylate polymer biocompatibility in the eyes of pilots who had sustained ocular injuries from polymethylmethacrylate (PMMA) shards when their aircraft canopies were shattered. However, it took nearly thirty years for ophthalmologists to embrace IOL implantation as a routine method for restoring vision in patients suffering from diseased or damaged natural crystalline lenses.
Early IOLs were made from PMMA because of its proven biocompatibility. Polymethylmethacrylate is a ridged polymer and requires a 5 mm to 7 mm incision. Incision size is directly related to patient trauma, discomfort and healing times. Moreover, incisions sizes in the 5 mm to 7 mm range generally require sutures further increasing procedural complexity and patent discomfort. Lens size dictates incision size and lens size is in turn determined by the size of the capsular sac and natural crystalline lens. Thus lenses made from a rigid polymer such as PMMA require an incision size at least as large as the minimum IOL dimension which is generally 5.5 mm on average.
In an effort to decrease incision size and corresponding patient discomfort, recovery time and procedural complexity, a number of IOL designs suitable for insertion through small incisions have been developed; most notably foldable IOLs. Foldable IOLs are made from non-rigid, or pliable polymers including hydrophobic acrylics, hydrophilic hydrogels, silicone elastomers and porcine collagen. Intraocular lenses made form these materials can be folded or rolled into implantable configurations having minimum dimensions suited for 3 mm incisions, or less.
Traditionally, IOLs have been exclusively used to restore vision to patients having damaged natural crystalline lenses or cataracts. These generally involved implanting a polymeric IOL into the capsular sac in the eye's posterior chamber after the damaged natural crystalline lens was surgically removed. Recently, refractive correction using IOLs in the phakic eye has grown in popularity as an option to refractive laser surgery. However, there are difficulties associated with implanting an IOL in the phakic eye that are not encountered when implanting a lens in the aphakic eye. The phakic eye is a substantially more reactive environment than the aphakic eye. Inflammatory reactions tend to be greater in the phakic eye resulting in a concomitant increase in damage to the eye caused by implanting intraocular lenses. Moreover, the presence of the natural lens in the phakic eye significantly reduces the space available for posterior chamber implantation. Thus, an IOL implanted into the posterior chamber of the phakic eye will directly contact the posterior surface of the natural crystalline lens. Under some circumstances this can result in permanent injury to the natural crystalline lens. Consequently, efforts to implant a refractive correcting IOLs into the eye's anterior chamber have been developed.
The anterior chamber of an eye is that area in front of the iris and behind the cornea. The iris separates the anterior chamber and the posterior chamber and thus IOLs implanted into the anterior chamber of the phakic eye rest against the iris not the natural crystalline lens. However, the phakic eye has a narrow anterior chamber thus lenses implanted in the anterior chamber must be thinner than those used in the posterior chamber. Moreover, it is desirable to minimize the incision size used to implant the anterior chamber IOL for the reasons discussed above. Consequently, the anterior chamber IOL must be at least as pliable as a posterior chamber IOL but must be thinner. Unfortunately, this combination of attributes has proven to be exceeding difficult to obtain. Lenses made thin and pliable enough to fit comfortably into the eye's anterior chamber lack the mechanical strength (resiliency) necessary to withstand casual contact or impact injuries such as those experienced in every day life. Anterior chamber IOLs made strong enough to resist incidental impact damage are generally thicker and thus must be inserted through larger incisions and are generally limited to minus refractive corrections, or extremely limited in the degree of positive correction possible.
Therefore, there is a need for biocompatible polymeric compositions that can be used to make an anterior chamber IOL that are thin and pliable enough to fit easily through small incisions, have sufficient mechanical strength to resist impact-related damage and can be made in a wide range of diopters sufficient to provide refractive correction for myopia, hyperopia, presbyopia and astigmatisms.